Merging families

[eventide]

I am documenting 2 families with common ancestors. I have 2 separate family files and I' marked the ancestors from one file and then, in the other file choose File > Import > Reunion Family File and select the "Marked people" option in the import window.

That worked - however I cannot seem to merge a source. There are 2 exact matches that it can't find a match for but instead is finding others to match against. Any way to just replace one source with another without merging?

thanks.